I would go with the 70. I'm building a sig 1/6 cub which recomends a .25 2 stroke and I put a .48 four stroke in it. I also beefed up the wings a bit and some other structural things as the engine is heavier.

I just built my cub with a saito .91. I had to trim the stock motor mount a little.

When using a 4stroke does anyone put insulators or something to soften the vibration?
I have test ran my plane but not flown her yet. I want to fly off a pond with floats on this cub so this is why I went with the .91.

I flew mine with floats for the first time. I must say the Saito .91has more then enough power. I was only able to get one flight in so not a lot of info but she seemed to come up at about 30 or so feet.

Maybe next week I can get some video. I do love this setup, she flies really nice with floats.

Images

I have a Saito .62 in my brother's H9 cub and it flies great with plenty of power even with floats. A .91 FS is way over kill in my opinion for a .40 size cub. Be careful as a broken stab or even a folded wing is a possibility at higher throttle. I have personally seen it happen with the H9 cubs when overpowered.

Good advice. I just float it around in the sky. I try and fly scale. I like the look and feel. But as for power I like having extra. Not ever owning one with floats I was not sure what to expect.
I have a road right at the end of the pond and might need to clear a car, its really not an issue but could be. So all the extra power is nice.
After building this and doing the run up for the first few times, I was really careful because I worried about the firewall breaking or failing.
But she has held together so far with just a few flights.